Various electrosurgical methods, the effect of which is based on a controlled denaturation of biological tissue, are in use.
For example, EP 1 862 137 A1 discloses a coagulation device comprising a generator, which feeds two electrodes, between which biological tissue is seized. During the coagulation, the tissue runs through a first phase I, during which the tissue impedance decreases considerably, and a second phase II, during which the tissue impedance increases again. To determine the tissue impedance, provision is made for a sensor circuit, which transmits a query signal, so as to determine the initial tissue impedance and so as to subsequently define a certain trajectory for the desired course of time of the tissue impedance. The query signal is formed by means of an electric pulse, by means of which a tissue characteristic is measured. The measured tissue characteristic can be energy, power, impedance, current, voltage, electric phase angle, reflected power or temperature.
U.S. Pat. No. 8,216,223 B2 also deals with the coagulation of tissue. The tissue impedance is initially measured during an HF activation of electrodes. Over the course of time, the minimum of the impedance is established. Starting at this point, a reference value curve is generated for the desired impedance increase and a target value is calculated for the impedance. Once the latter has been reached, the HF generator is turned off. The turn-off is followed by a cooling phase, the length of which is also provided by the reference value curve. The fusion is concluded with the end of the cooling phase.
The thermofusion according to U.S. Pat. No. 8,034,049 B2 is also controlled by means of the initial tissue impedance. In phase I of the thermofusion, the course of the impedance is measured in response to current, which is kept constant, for example. The initial impedance, the decrease of the impedance, the minimum of the impedance or the increase of the impedance are derived from this. Other activation parameters are generated from this information.
EP 2 213 255 B1 describes the control of the energy in response to a thermofusion. A state variable SV, which indicates the decrease or increase of the impedance, is generated for this purpose. A reference value trajectory is provided for the impedance. The energy input is controlled such that the desired chronological course of the impedance is approximated. For this purpose, the energy input is coupled or countercoupled as a function of the state variables SV to the impedance.
EP 2 394 593 A1 describes the measuring of the impedance during the thermofusion. Provision is made to check, whether, after a certain minimum time has lapsed, a minimum impedance has been reached. As soon as this is the case, the activation is concluded.
U.S. Pat. No. 6,733,498 B2 discloses a method for thermofusion, in the case of which the chronological course of the tissue impedance is determined during the application of HF voltage. The end of the first phase and the duration of the second phase are defined accordingly by means of the course of the impedance.
U.S. Pat. No. 8,147,485 B2 also uses the monitoring of the tissue impedance for regulating the thermofusion. An impedance trajectory is calculated from the minimum of the tissue impedance and the impedance increase.
U.S. 2010/0179563 A1 and U.S. 2011/0160725 A1 also determine the tissue impedance or the change thereof for controlling or regulating the electrosurgical process.
The local state of tissue is characterized by the local specific tissue impedance. Even though the determination of the impedance between two electrodes provides an indication for the state and thus for the treatment progress of the tissue as a whole, the local specific tissue impedance, however, is not determined. This can lead to incorrect conclusions in the case of inhomogeneous tissue.